Mitochondrial cytochrome oxidase and ATPase have been isolated from Xenopus and Sea Urchins respectively. Cytochrome oxidase consists of 9 peptides while ATPase contains 7 peptides. Two peptides of each enzyme complex are made on mitochondrial ribosomes. Attempts are being made to determine the site of transcription, mitochondrial or nuclear, of mRNAs for the mitochondrially made peptides. Two approaches are being used to answer this question: 1. Mitochondrially made peptides from two species of Xenopus X. laevis and X. mulleri, are being characterized to determine whether differences can be detected between corresponding peptides. Hybrids of these species will then be produced and the mode of inheritance of these peptides will be determined. If a given cytochrome oxidase peptide shows a Mendelian pattern of inheritance it must be encoded in the nucleus. If the pattern of inheritance is cytoplasmic its mRNA must be transcribed from mitochondrial DNA. 2. Mitochondrial poly A-RNA has been isolated from sea urchins. It will be tested for ability to stimulate synthesis of ATPase peptides in a bacterial cell-free system. If such peptides are produced the specific poly A-RNAs coding for each peptide will be isolated and its site of transcription identified by RNA-DNA hybridization.